Large scale economic conversion of rotary hydrokinetic power must address the challenge of extremely slow rotation with extremely high torque. Modern wind power conversion shares this challenge, but the higher viscosity of water imposes an even lower functional limit on turbine blade tip speed. Engineering designs to meet this challenge are high in cost, including large gear boxes, direct drive generating units, or hydraulic transmissions. This challenge increases with turbine diameter.
The present invention is a value engineering solution to enable economic deployment of large diameter horizontal axis hydrokinetic turbines. A large freewheeling turbine incorporates secondary turbines at the tips of its blades where localized energy density is greatest. In effect, this arrangement employs the ambient fluid as a planetary gear ring. The required generating units are economical high speed type.
The secondary turbines are positioned in way of the induced primary rotor blade tip vortices to partially recover that rotational energy. In this arrangement, vortex energy recovery provides the further benefit of dissipating vortex drag, which is otherwise a significant drag upon free stream energy conversion rotors.
Because no torque is translated through the primary turbine shaft, there is significant structural savings in avoided reaction torque support.
In the prior art, the Minesto Company has proposed a kite-turbine that also incorporates a secondary turbine. It flies in a dynamically controlled figure-eight path at reported speeds up to ten times the ambient current speed. In comparison, the present invention does not expend energy in tether motion and partially recovers induced wing tip vortex energy that is otherwise a source of drag.
The prior art describes means to recover wing-tip vortex energy in fixed wing aircraft, for example U.S. Pat. No. 4,428,711 to Archer, U.S. Pat. No. 4,917,332 to Patterson, U.S. Pat. No. 5,100,085 to Rubbert, U.S. Pat. No. 5,150,859 to Ransick, U.S. Pat. No. 5702,071 to Kroll et al., U.S. Pat. No. 5,918,835 to Gerhardt, and U.S. Pat. No. 5,934,612 to Gerhardt. For wind energy conversion, the prior art also describes means to convert induced vortex energy from other non-rotating forms, for example U.S. Pat. No 4,045,144 to Loth, U.S. Pat. No 4,105,362 to Sforza, and U.S. Pat. No. 7,131,812 to Brueckner. The present invention is distinguished by the combination of primary and secondary rotors in a single device.